@tilekarnilesh 

try this

(function executeRule(current, previous /*null when async*/) {

    // Get the Requested Item (RITM) record
    var ritmRecord = current.request_item.getRefRecord();

    // Access the variable 'project_category' from the RITM
    var variableValue = ritmRecord.variables.project_category;

    // If value is 'Standard', set due date to 5 business (week) days later
    if (variableValue == 'Standard') {

        var businessDaysToAdd = 5;
        var daysAdded = 0;

        var gdt = new GlideDateTime();

        // Start with today's date and time
        var startDate = new GlideDateTime();
        gdt.setValue(startDate.getValue()); // Retain current date and time

        while (daysAdded < businessDaysToAdd) {
            gdt.addDaysUTC(1); // Add 1 calendar day at a time

            // Get the date portion in UTC and determine the weekday
            var gd = new GlideDate();
            gd.setValue(gdt.getDate()); // getDate() gives UTC date portion
            var dayOfWeek = gd.getDayOfWeek(); // 1 = Sunday, 7 = Saturday (UTC-based)

            // Skip weekends (Saturday = 6, Sunday = 7), count only weekdays
            if (dayOfWeek != 6 && dayOfWeek != 7) {
                daysAdded++;
            }
        }

        // Set the due date in the current record
        current.due_date = gdt;
    }

})(current, previous);

It worked for me in background scripts

  var businessDaysToAdd = 5;
  var daysAdded = 0;

  var gdt = new GlideDateTime();
  gs.info('now time' + gdt);
  // Start with today's date and time
  var startDate = new GlideDateTime();
  gdt.setValue(startDate.getValue()); // Retain current date and time

  while (daysAdded < businessDaysToAdd) {
      gdt.addDaysUTC(1); // Add 1 calendar day at a time

      // Get the date portion in UTC and determine the weekday
      var gd = new GlideDate();
      gd.setValue(gdt.getDate()); // getDate() gives UTC date portion
      var dayOfWeek = gd.getDayOfWeek(); // 1 = Sunday, 7 = Saturday (UTC-based)

      // Skip weekends (Saturday = 6, Sunday = 7), count only weekdays
      if (dayOfWeek != 6 && dayOfWeek != 7) {
          daysAdded++;
      }
  }
  gs.info('Final date' + gdt);

Output:

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